Dosimetric measurements with a brain equivalent plastic walled ionization chamber in an epithermal neutron beam.

The tissue substitute A-181 plastic, which has an elemental composition matching both the constituent hydrogen and nitrogen of brain tissue, was assessed for dosimetry in boron neutron capture therapy (BNCT). The sensitivity of an A-181 walled ionization chamber relative to photons for all neutrons in a clinical epithermal beam was calculated to vary between 0.79 +/- 0.04 in-air and 0.95 +/- 0.01 at depths of 4 cm and greater in-phantom. Differences in the total neutron doses measured with A-150 and A-181 plastic-walled chambers were attributed, within experimental error, to the dose produced by thermal neutron capture reactions from the different concentrations of nitrogen in the two tissue substitutes. The response of the A-181 chamber was converted to total neutron dose with an uncertainty increasing with depth in-phantom from 13 to 23% the magnitude of which is determined by the subtraction of a relatively large photon dose. The use of A-181 in place of A-150 plastic will no longer require partitioning the measured neutron dose by energy and should simplify dose reporting in BNCT.